Charged particles beam systems, such as electron beam systems and ion beam systems, can form images by detecting secondary or backscattered electrons or ions that are emitted when the primary beam impacts the work piece surface. For many years, scanning electron microscopes and focused ion beam systems have used a scintillator-photomultiplier detector, known as an Everhart-Thornley detector or simply an “ET detector,” or ETD to detect secondary electrons. Other types of electron detectors include a channel detection electron multiplier, (sometimes referred to as a continuous dynode electron multiplier or CDEM) and a multichannel plate (MCP).
FIG. 1A shows schematically a typical prior art dual beam system 100 having an electron column 102 and a focused ion beam column 104 for operating on a sample 105 in a vacuum chamber 107 and using a conventional ET detector 140. When primary electron beam 120 impacts sample 105, secondary electrons 142 are emitted and are accelerated by an applied voltage toward a grid 106 and impact a scintillator 108, which is composed of a material, (such as a phosphor, a light-emitting plastic or a garnet oxide) that emits light (called cathodoluminescence) when impacted by charged particles such as electrons 142. The light is typically conducted by a rigid light pipe 110, a solid plastic or glass rod passing through a sealed port in the specimen chamber, through a transparent window 112 in the vacuum chamber to a photomultiplier tube (PMT) 114 that is positioned outside the vacuum chamber. In PMT 114, the light causes the emission of electrons, which impacts dynodes in the photomultiplier tube, which cause the emission of additional electrons.
The additional electrons impact the dynodes of PM tube 110, creating a cascade of electrons that results in an amplified signal. The electrons are collected by an electrode at the end of the PM tube. The electrons constitute an electric current which is conducted by a conductor 130 to imaging circuitry 132. The magnitude of the current is used to determine the brightness of a point on the image corresponding to the point on the work piece where the beam is impacting. As the beam scans the work piece, the image is built up, point by point.
FIG. 1B shows a schematic drawing of the vacuum chamber of the prior art SEM of FIG. 1A. It shows the final lens 102, the so-named objective lens of an electron column and a portion of an ET detector 160 that combines a through-the-lens secondary electron detector 162 and a non-through-the-lens secondary electron detector 164. The through-the-lens secondary electron detector converts electrons to photons inside the structure of the objective lens and transports the photons via a light guide 162 to a PM tube positioned outside the vacuum chamber (not shown). The non-through-the-lens secondary electron detector shows a grid 164 sucking the secondary electrons to a fluorescent screen within housing 166, where the electrons are converted to photons, which in turn are guided to the PM tube outside the vacuum chamber.
FIG. 2A shows schematically a dual beam system similar to that of FIG. 1A, but using a CDEM detector 206 having an input grid 202 and spiral channels (not shown) that amplify the electron signal when its walls are impacted by electrons by producing additional electrons.
FIG. 2B shows a schematic drawing of the vacuum chamber of the prior art dual beam system of FIG. 2A. It shows a CDEM 206 by the objective lens 252 of an electron column and a gas injection system 254 near CDEM 206.
In some dual beam systems, there are a multitude of accessories competing for space in sample chamber 107 near the point where the beam 120 impacts the work piece 105. Accessories may include, for example, one or more gas injection nozzles 254, one or more micromanipulators (not shown), a charge neutralization electron flood gun, and different types of detectors, such as a backscattered electron detector (not shown), x-ray detectors, and a secondary electron detector. Ion column 104 and electron column 102 are preferably positioned close to the work piece to enhance resolution.
When an electron or ion in a primary beam impacts a work piece surface, the emitted secondary electrons are distributed substantially equally in all directions throughout a hemisphere over the work piece. The further the scintillator is from the work piece, the smaller the fraction of the emitted electrons that will impact the scintillator. A positive electric charge on the detector relative to the sample increases the number of secondary electrons that impact the scintillator, but it is thought that only about fifteen to twenty percent of the secondary electrons emitted from the sample actually reach a conventional ET detector.
The secondary electron detector is typically positioned with the scintillator within the sample chamber 107, sufficiently far from the work piece 105 to allow the charged particle beam columns and other accessories to fit near the sample or work piece. Because the light tube 110 must lead from the scintillator to the window 112 in the vacuum chamber, the scintillator cannot be repositioned within the chamber without redesigning the light tube and/or changing the location of the window in the vacuum chamber. Thus, users of dual beam systems have limited flexibility in adapting their systems for new accessories and new applications. Moreover, at each interface in the light path, some of the light is lost. Conducting the light to the window, through the window, and then into the PM tube results in significant loss of light.
CDEMs and MCPs amplify the electron signal directly, without converting the signal to light and conducting the light outside of the vacuum chamber. Thus, a CDEM or MCP can be positioned entirely within a sample vacuum chamber. CDEMs and MCPs use an activated surface to amplify the electron signal. This surface degrades under charged particle stimulation, particularly if beam-activated chemicals, which are used in many deposition and etching processes, are introduced into the chamber. The surface also is prone to degradation when the chamber is vented for sample exchange.
In some applications, CDEMs can only function for a month or two before they need to be replaced. Thus, CDEMs are not efficient in many applications, in which many chemicals are used. CDEMs also have lower resolution than ETDs
Thus, the CDEM is typically placed in a good position for signal collection, but has limited lifetime. The ETD has good lifetime but is mounted in the wrong place for good signal collection and loses signal as the light moves through coupled components to outside the vacuum chamber.
Ion-to-electron converters can be used with ETDs to detect ions and electrons replace CDEM or MCP and can eliminate the lifetime issues with those detectors, but with a PMT outside the vacuum chamber, still require complex, rigid assemblies with vacuum feedthrough ports in the right locations on the chamber wall relative to the desired placed for the detector. This detector has to be re-engineered in a major way for every different system.
Designer and users of dual beam instruments are acutely aware of the congested space close to the sample due to the presence of an electron column, ion column, gas injectors, sample probes, other detectors, such as backscatter electron detectors (BSD), x-ray detectors for energy dispersive spectroscopy (EDX), electron backscatter diffraction detectors (EBSD), and cathodoluminescence (CL) detectors. The inflexibility of positioning the ETD makes it difficult to reconfigure the instrument to move other devices close to the sample for particular applications.
The market need for both flexibility and long life have been known, but was not solved. The present invention solves the limited lifetime issue of the CDEM/MCP while retaining the flexibility of the mounting arrangement.